1. Field of the Invention
The present invention relates to a multi-piece golf ball with improved carry without sacrificing properties such as spin performance, durability, coefficient of restitution, the feel when the ball is hit, and the like.
2. Description of the Background Art
Basic properties required of a golf ball include carry, spin performance, durability, the feel when the ball is hit, and the like. In order to increase the carry, a golf ball preferably has a high moment of inertia.
For example, a multi-piece golf ball with an intermediate layer between the core and the cover has a larger specific gravity in the intermediate layer than the core so as to achieve a higher moment of inertia of the ball. When a golf ball with such a structure is hit by the golf club, it generates spins and thereby produces a lift that is normal to the trajectory of the ball. While the ball flies upward upon being shot, the lift in its horizontal component functions to exert a negative force to the ball with respect to the direction in which the ball travels, thereby decreasing the speed of the ball, which was high immediately after being shot. On the other hand, in the lift caused by the spin of the ball that has passed the highest point of the trajectory and is flying downwards, its horizontal component affects the ball as a positive force with respect to the direction in which the ball travels. Thus, it is preferable to have a greater lift when the ball is flying downwards in order to increase the carry.
Consequently, to increase the carry of a golf ball, it is preferable to have a low spin rate when the ball is flying upwards upon being shot and to prevent the spin rate from decreasing while the ball is flying downwards. Accordingly, it is preferred to design a golf ball to have a higher moment of inertia.
In order to increase the moment of inertial of a golf ball, it has been proposed to decrease the weight of the core. The core of such a conventional golf ball is composed of a molded rubber vulcanizate mixed with a light weight filler, a molded resin mixed with a light weight filler, or a molded foam of a rubber or resin (see European Patent Publication No. 0600721). Unfortunately, the use of this technique causes the intermediate layer to contain a large amount of filler, resulting in a lower coefficient of restitution of the golf ball which results, in turn, in a smaller carry.
Also, a foam may be used for the core of a golf ball to decrease the specific gravity of the core, while providing a cover or an intermediate layer with high specific gravity in order to increase the moment of inertia (see, for example, U.S. Pat. No. 6,010,412). Although this technique provides increased moment of inertia of a golf ball, it suffered from a lower coefficient of restitution and lower durability of the golf ball.
Further, the technique is known to provide a golf ball composed of a hollow core and a cover layer, where the hollow core is constructed of a hollow portion with a diameter of 5 to 30 mm and the remaining portion of the core forming the outer layer of the hollow core, the outer layer being composed of a molded vulcanizate of a rubber composition that contains a base rubber, a metallic salt of an unsaturated carboxylic acid, an organic peroxide and a filler, in order to provide increased moment of inertia, greater shot angle when the ball is hit and longer carry, and to improve the soft feel (Japanese Patent Laying-Open No. 9-308709). Such a golf ball, however, has the disadvantage of a lower coefficient of restitution of the golf ball and a poor durability.